1. Field of the Invention
This invention relates to a pressure sensor device for detecting gas or liquid pressure fluctuations and oscillating an electric signal, the pressure sensor device being used, for example, to monitor the air pressure of a tire.
2. Description of the Related Art
A device of a type that detects the fluctuation of applied pressure as a change in oscillation frequency has been conventionally used as a pressure sensor device for detecting gas or liquid pressure fluctuations.
As an example of this type of conventional pressure sensor device, there is known a pressure sensor device having a structure in which surface acoustic wave elements 54 and a surface acoustic wave element 57 each of which is formed of a pectinate electrode are disposed on a piezoelectric substrate 51, and in which a region where the surface acoustic wave element 54 is formed is set to be thinner in thickness than a region where the surface acoustic wave element 57 is formed, as shown in FIG. 17 and FIG. 18 (see Japanese Unexamined Patent Publication No. 61-82130 (1986), for example).
In this conventional pressure sensor device, the surface stress of the piezoelectric substrate 51 is changed by having received pressure in the surface acoustic wave element 54 formed in the region whose thickness is made thinner, and the acoustic velocity of surface acoustic waves varies, and, at the same time, a distance between the electrodes of the surface acoustic wave elements 54 varies. Accordingly, the resonance frequency of the surface acoustic wave elements 54 varies, and pressure can be detected by a change in the resonance frequency.
The conventional pressure sensor device also has a function to make temperature compensation in accordance with a change in the resonance frequency of the surface acoustic wave element 57 disposed on the same piezoelectric substrate.
However, the conventional pressure sensor device has a conventional problem in the fact that the surface acoustic wave element 57 and the surface acoustic wave element 54 are formed in the same area on the surface of the piezoelectric substrate 51, and hence the area of the piezoelectric substrate is enlarged, thus making it difficult to achieve a size reduction. The conventional pressure sensor device has another conventional problem in the fact that, although the piezoelectric substrate 51 partly has a thin part, a large area for the thin part cannot be secured when the pressure sensor device must be reduced, for example, in size, and therefore the amount of deformation obtained by receiving pressure is small, and high sensitivity cannot be obtained as a pressure sensor device.
It is an object of the present invention to provide a pressure sensor device that can improve its sensitivity by enlarging the deformation of a surface acoustic wave element when pressure is received, that can be reduced in size, and that has high reliability.